1. Field of the Invention
The present invention relates to optical disks and, more particularly, to methods and apparatus for recording information thereon.
2. Description Relative to the Prior Art
An optical disk is a record device comprised of a layer (or layers) of recording material coated on a disk-shaped substrate. During the recording process, the optical disk is spun and a recording spot formed by focusing a laser beam on the optical disk surface is modulated in accordance with information to be recorded while being scanned radially across the spinning optical disk. As a result, information is recorded on the disk along a spiral track extending between outer and inner disk radii. Alternatively, it is known to scan the recording spot across the optical disk in such a manner as to cause information to be recorded along concentric circular tracks. The term radial format recording is used hereinafter to refer to such recording formats, as well as to variations thereof.
It is often convenient to record the same amount of information (e.g., one television frame or field) or the same bandwidth of information per revolution of the disk, irrespective of the radial position on the disk. As a result, the in-track spatial frequency (i.e., reciprocal length) of recorded information marks increases with decreasing radial position because the record track length corresponding to a revolution of the disk is proportional to its radial position. As an illustrative example, assume that information is recorded on an optical disk at an outer radial position of 100 mm and an inner radial position of 50 mm. If the information recorded at the outer radial position is in the form of marks which are 1.2 microns in length and which are spaced apart by 0.8 microns, then this same information should be recorded at the inner radial position in the form of marks which are 0.6 microns in length and which are spaced apart by 0.4 microns. As will be seen from the discussion which follows, this radial dependence of the in-track spatial frequency of recorded information leads to problems upon playback.
In the playback process, a playback spot is formed on the optical disk surface and is made to follow the track of recorded information. The interaction between the playback spot and the recorded information causes the playback spot to be "modulated" (as described in detail below) in accordance with the recorded information. By directing the "modulated" playback spot onto a detector, an electrical signal is produced which is representative of the recorded information. Ideally, the information content of the electrical signal produced upon playback should be the same as the originally recorded signal.
In actual practice, the scaling of the spatial frequency of recorded information marks as a function of disk radius leads to distortion upon playback because of the modulation transfer function of the playback optical system. Generally, such an optical system is diffraction limited and, as is well known, the modulation transfer function decreases as the spatial frequencies being imaged increase. As a result, the signal produced by playing back information recorded at inner radial positions (higher spatial frequencies) is attenuated relative to the signal corresponding to playback of information recorded at outer radial positions (lower spatial frequencies). The signal-to-noise ratio of the playback signal, therefore, decreases with decreasing radial position. A method around this problem is disclosed in U.S. Pat. No. 4,142,210 wherein information is recorded using a single mark size irrespective of the radial position of recording. This method, however, results in a significantly lower packing density of recorded information. An alternative method for compensating for the modulation transfer function of the playback optical system is disclosed in U.S. Pat. No. 4,118,734 which teaches the recording of information tracks which progressively increase in width with decreasing disk radius. This method also suffers from the disadvantage of significantly reducing the packing density of recorded information, as well as requiring mechanical apparatus for changing the size of the recording spot. It has also been proposed to correct for this source of distortion by decreasing the size of the playback spot at inner radial positions. See U.S. Pat. No. 4,157,568. As in the previous case, this method suffers from the disadvantage of requiring mechanical apparatus for changing spot size.
Insofar as the lengths of recorded information marks are concerned, it is also known that it is desirable to record information marks having an average information mark duty cycle of 50 percent. The information mark duty cycle is defined as the ratio of the mark length to the mark-to-mark center spacing. See, for example, "Intermodulation and Moire Effects In Optical Video Recording", by M. R. de Haan, Philips Res. Repts., Vol. 32, pp. 436-59, 1977. In particular, recording information at an average duty cycle of 50 percent minimizes and even eliminates second harmonic distortion. In terms of the recorded information marks (i.e., bumps, pits, craters, density images, etc.), an average duty cycle of 50 percent means that the average length of recorded information marks equals the average length of the spaces between such marks.